The invention relates to the manufacture of resin-treated photographic paper film bases with improved durability.
Photographic paper film bases having a high resistance to the effect of photographic processing solutions include a base paper which is treated on both sides with resin, as is known. Such resin-treated papers are preferably manufactured by extrusion treatment with the aid of a fishtail die, whereby a thermoplastic polyolefin resin, e.g. polyethylene, is applied on both sides of the web of paper. As a rule, the front resin coating contains at least a white pigment to enhance good image or picture definition. It may also contain fluorescent dyestuffs (white toners) and shading dyes in order to obtain an optical impression of increased whiteness of the coated paper and to adapt it to the requirements of the market dependent on taste.
The process is also known of preparing resin-coated base materials by an oxidizing primary treatment, e.g. by means of coronal or flame treatment, in such a way that they can subsequently be coated with photosensitive emulsion layers which adhere to the underlayer. For the front resin layers facing the photosensitive layers, high-pressure polyethylenes are preferred which have a lesser thickness and a branched molecular structure.
Although polyolefins are generally considered to be relatively stable, and branched polyethylenes in particular, just like other resins with tertiary C-atoms, they are subject to separation which appears during the extrusion process and also, above all, from the action of light on the coated material. Therefore, stabilizers and/or antioxidants are added to such polyethylenes, as are usual in the case of oxidation-sensitive resins, which offer some protection against this undesirable separation.
There are a multitude of stabilizers and antioxidants which vary chemically, which are currently usually combined to form effective mixtures in accordance with the various reaction mechanisms of the polymer breakdown. Suitable examples of such compounds are specified in the main in published German patent application No. 2,160,463.
In this published patent application, the determining disadvantage of all stabilizing and antioxidant active additives is also described. The additives are prone to efflorescence. Consequently, with the usage of stabilizers and antioxidants in the paper coating resins, the adhesion of the resin to the base paper is impaired to such an extent that, with further treatment processes, a separation of the layer may occur. Furthermore, in the case of antioxidants which contain sulphur in the molecule, e.g. thiobisphenol, decomposition products emerge which produce a haze in the overlying photographic layers.
One solution to these problems is suggested in published German patent application No. 2,160,463. That published patent application suggests that the antioxidants and stabilizers should not be incorporated into the resin layer, but instead, they should be worked into the base paper. From there they will diffuse into the resin coat to stabilize it during normal usage of the paper product.
One disadvantage of this process,, however, is that the pigmented resin material is not protected during the extrusion procedure with its attendant heavy mechanical and thermal demand. A further disadvantage is the fact that the proportion of stabilizers and/or antioxidants acquired at normal temperatures through diffusion into the resin layer is very slight. In actual fact, the effect of the process described in published German patent application No. 2,160,463 is so limited that the durability of the photos, which are exposed to a constantly changing influence of light, heat and humidity in showcases, is only increased by some 10 to 60%. Example 1 of that application demonstrates an improvement in the durability of from 112 to 184 hours. Example 2 only produces an improvement of the durability from 10 days to 11 days. In the practice of photographic publicity, on the other hand, it is desirable that photographs should be able to remain in showcases for several months without damage. Embodiments of this invention, thus, seek to create resin coated paper bases for photographic films, which are not only free from the described disadvantages of impairment of adhesion of the resin coat, but are also considerably improved vis-a-vis the durability of the photographs in showcases.